1. Field of the Invention
The present invention relates to devices for treating fuel vapor, in particular, fuel vapor processors for trapping fuel vapor, which has vaporized in a fuel tank, in a canister and recovering the fuel vapor from the canister to the fuel tank due to a suction device.
2. Description of the Related Art
A gas vehicle is equipped with a fuel vapor processor for preventing fuel vapor vaporized in a fuel tank from flowing into the atmosphere for environmental protection or the like. The fuel vapor processor includes a canister filled with adsorbent and temporally traps the fuel vapor by adsorbing the fuel vapor onto the adsorbent. As for a conventional fuel vapor processor shown in Japanese Laid-Open Patent Publication No. 10-259765, the canister and an air intake pipe for an engine are communicated with each other via a purge pipe, and the fuel vapor trapped in the canister is removed due to negative pressure generated in the air intake pipe while driving the engine. The removed fuel vapor is delivered to the air intake pipe through the purge pipe and then is used for engine combustion. In addition, with respect to Japanese Laid-Open Patent Publication No. 10-259765, an air pipe communicating the canister with the atmosphere has an air pipe valve for opening and closing the air pipe, and the fuel tank has a pressure sensor for detecting inner pressure of the fuel tank, and thus back-purge is allowable. In particular, when the inner pressure of the fuel tank reaches allowable lower limit for negative pressure due to decrease in fuel temperature during parking or the like, a controller (ECU) repeatedly opens and closes the air pipe valve such that fresh air (ambient air) is led into the canister while keeping negative pressure in the fuel tank. Periods for opening and closing the air pipe valve are controlled depending on alteration of pressure in the fuel tank.
In this way, the fuel vapor processor in Japanese Laid-Open Patent Publication No. 10-259765 is configured as “purge system” for purging and delivering the fuel vapor to the air intake pipe due to negative pressure in the air intake pipe. Recently, the number of vehicles equipped with “idling stop system” or “hybrid system”, e.g., for reducing fuel consumption or exhaust fumes increases. As for the vehicles with such system, opportunities for generating negative pressure in the air intake pipe decrease. For example, “idling stop system” stops the engine during stopping at a red light or the like, so that it is not able to utilize negative pressure in the air intake pipe during stopping. Whereas, “hybrid system” stops the engine during drive mode by an electric motor, so that it is not able to utilize negative pressure in the air intake pipe during such drive mode. Thus, opportunities for removing the fuel vapor trapped in the canister decrease, so that the adsorbent in the canister cannot be renewed adequately. Therefore, it is difficult to appropriately treat the fuel vapor. In addition, some of vehicles equipped with “direct injection fuel engine” for directly injecting fuel into a combustion chamber do not have any throttle valve, so that it is difficult for this type vehicle to purge the fuel vapor trapped in the canister into the air intake pipe.
Therefore, fuel vapor processors including “purge-less evaporation system” without purging the fuel vapor to the air intake pipe have been provided. Some of such fuel vapor processors are disclosed in, e.g., Japanese Laid-Open Patent Publications No. 2003-314381 and No. 2000-282966. Each of these fuel vapor processors has a canister for trapping fuel vapor generated in a fuel tank, a vapor pipe for delivering the fuel vapor from the fuel tank to the canister, a recovery pipe for recovering the fuel vapor removed from the canister into the fuel tank, an air pipe communicating the canister with the atmosphere, a vacuum pump (suction device) disposed on the recovery pipe for removing the fuel vapor trapped in the canister, a vapor pipe valve for opening and closing the vapor pipe, and an air pipe valve for opening and closing the air pipe. During removal of the fuel vapor due to the vacuum pump, the vapor pipe valve and the air pipe valve are closed such that the fuel vapor is removed while keeping negative pressure in the canister. That is, during removal operation of the fuel vapor, an inner space of the canister except the recovery pipe is hermetically closed.
In detail, Japanese Laid-Open Patent Publication No. 2003-314381 discloses a separation membrane disposed on the recovery pipe downstream of the vacuum pump for separating gas including the fuel vapor removed from the canister into fuel components and air in order to obtain concentrated gas with increased concentration of the fuel vapor and diluted gas with decreased concentration of the fuel vapor. The concentrated gas separated by the separation membrane is recovered to the fuel tank while cooling the concentrated gas due to a cooler including a peltier element in order to accelerate devolatilization. Whereas, the diluted gas separated by the separation membrane can be returned into the canister through a return pipe. Japanese Laid-Open Patent Publication No. 2000-282966 discloses a dispersion device disposed on an end of the recovery pipe. The recovered fuel vapor is dispersed into the fuel in order to increase solubility of the fuel vapor due to this dispersion device. Japanese Laid-Open Patent Publication No. 2009-000646 discloses a fuel vapor processor for recovering fuel vapor generated in a fuel tank by a pressure swing adsorption device having two towers, however the disclosed fuel vapor processor is used for gas station and is not mounted on a vehicle.
The fuel vapor processor disclosed in Japanese Laid-Open Patent Publication No. 10-259765 utilizes negative pressure in the air intake pipe, so that it would be difficult to mount the processor on a vehicle having “idling stop system” or “hybrid system”. The fuel vapor processors disclosed in either Japanese Laid-Open Patent Publication No. 2003-314381 or No. 2000-282966 recover the fuel vapor into the fuel tank due to the vacuum pump, so that the fuel vapor processors can be mounted on any type vehicles. During recovery operation for the fuel vapor by the vacuum pump, negative pressure is kept in the canister. Thus, inner space of the fuel vapor processor except the recovery pipe is hermetically closed, so that ambient air does not flow into the canister during removal operation. So, while inner pressure of the canister gradually decreases, gas flows in the canister, so that the fuel vapor can be desorbed. However, when the inner pressure of the canister reaches a limit for pressure reduction, gas hardly flows in the canister, so that fuel vapor cannot be removed any more. Therefore, desorption efficiency (desorption amount) is not enough.
According to a result of examination as for desorption efficiency of fuel vapor by inventors of this invention, it has been found that desorption efficiency in a condition that negative pressure is kept in the canister and fresh air flows into the canister is better than that of a case that fresh air does not flow into the canister. In particular, gas containing 50% butane was supplied to a canister filled with 3 litter of activated carbon as adsorbent such that butane adsorbs over breakthrough of 2 gram. Then, desorption amounts either in a case that air (fresh air) flew into the canister or in a case that air did not flow into the canister were measured while keeping negative pressure in the canister according to conditions shown in Table 1 due to a vacuum pump. Here, air flew into the canister at 1.5 L/min for 30 min (total 45 litter). Results of such examination are shown in Table 1.
TABLE 1ConditionAirflowNegative Pressure (kPa)Desorption amount (g)1◯0 (atmospheric pressure)39.72◯−4059.93◯−7078.44◯−8598.65×−706.0
The results in Table 1 show that in a case that fuel vapor was desorbed while flowing fresh air into the canister, the lower inner pressure of the canister was, the higher desorption efficiency was. In particular, compared with condition 1 where the fuel vapor was desorbed at atmospheric pressure, desorption amount was about 1.5 times in condition 2 where −40 kPa of negative pressure was provided to the canister, desorption amount was about 2 times in condition 3 where −70 kPa of negative pressure was provided to the canister, and desorption amount is about 2.5 times in condition 4 where −85 kPa of negative pressure was provided to the canister. On the other hand, a result of condition 5 shows that desorption amount was drastic low in a condition that −70 kPa of negative pressure was provided to the canister and fresh air did not flow into the canister compared with a condition that the fuel vapor was desorbed while flowing fresh air into the canister.
In disclosure of Japanese Laid-Open Patent Publication No. 10-259465, back-purge is carried out due to negative pressure when inner pressure of the fuel tank decreases by fuel temperature reduction during parking, and pressure is controlled such that negative pressure is kept in the fuel tank while flowing fresh air into the canister. However, a suction device such as vacuum pump is not provided, so that when the inner pressure of the fuel tank reaches allowable lower limit for negative pressure, back-purge takes place.
In Japanese Laid-Open Patent Publication No. 2003-314381, a separation membrane, a fuel vapor cooler, a canister heater and the like are provided in order to improve treatment efficiency and treatment capacity. However, peltier element is used for the cooler, and piezo element is used for the heater. Thus, the number of members required for power distribution increases, so that the numbers of members such as distributing cable and assembly steps increase, and consumed power and loss during power transmission also increase. In addition, separation efficiency by the separation membrane is not 100%, so that diluted gas separated by the separation membrane would include components of fuel. Thus, there is a need for improved fuel vapor processors mounted in a vehicle for efficiently removing the fuel vapor trapped in the canister.